1. Field of the Invention
The present invention relates to a semiconductor device provided with improved adhesion between a low-dielectric-constant material and a semiconductor.
2. Description of the Related Art
Power consumption of a vertical cavity surface emitting laser (VCSEL) is lower than that of an edge-emitting laser diode, and the VCSEL is possible to be directly modulated. Therefore, the VCSEL is used as a cheap light source for optical communication in recent years.
A VCSEL, generally, has a columnar mesa in which a lower DBR layer, a lower spacer layer, an active layer, an upper spacer layer, an upper DBR layer, and a contact layer are stacked in this order on a substrate. Either the lower DBR layer or the upper DBR layer is provided with a current narrowing layer having a structure that a current injection region is narrowed in order to increase the efficient of injecting current to the active layer and to decrease threshold current. An electrode is provided for each of the top face of the mesa and the rear face of the substrate. In the VCSEL, current injected from the electrode is narrowed by the current narrowing layer and then injected to the active layer, thereby generating light by recombination of electrons and holes. The light is reflected by the lower DBR layer and the upper DBR layer, laser oscillation occurs at a predetermined wavelength, and the resultant light is emitted as a laser beam from the top of the mesa.
The diameter of the mesa is at most tens and the area of the electrode on the mesa is extremely narrow. Due to this, an electrode pad for wire bonding, which is electrically connected to the electrode on the mesa is formed around the mesa. However, parasitic capacitance is generated between the electrode pad and the lower DBR layer. For direct modulation at high speed, the parasitic capacitance has to be suppressed, and a device such as insertion of a low-dielectric-constant material below the electrode pad is necessary.
The characteristics necessary for the low-dielectric-constant material are the following three points: (1) excellent heat resistance and moisture resistance at low dielectric constant, (2) easy formation of a thick film by spin coating or the like, and (3) easy patterning. Polyimide is a representative low-dielectric-constant material having all of the characteristics and is a common material formed below the electrode pad and a wiring in order to reduce parasitic capacitance.